Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations

We report an ab initio study of the semiconducting Mg2X (with X = Si, Ge) compounds and in particular we analyze the formation energy of the different point defects with the aim to understand the intrinsic doping mechanisms. We find that the formation energy of Mg2Ge is 50 % larger than the one of Mg2Si, in agreement with the experimental tendency. From the study of the stability and the electronic properties of the most stable defects taking into account the growth conditions, we show that the main reason for the n-doping in these materials comes from interstitial magnesium defects. Conversely, since other defects acting like acceptors such as Mg vacancies or multivacancies are more stable in Mg2Ge than in Mg2Si, this explains why Mg2Ge can be of n or p type, contrary to Mg2Si. The finding that the most stable defects are different in Mg2Si and Mg2Ge and depend on the growth conditions is important and must be taken into account in the search of the optimal doping to improve the thermoelectric properties of these materials.Comment: 25 pages, 6 Table

Development of a regional-scale library of near infrared reflectance soil spectra for alternative assessment of soil characteristics in Southern Belgium

In Walloon Region (Southern Belgium), five routine soil laboratories are grouped within a network promoting a better quality in analysis (www.requasud.be). The harmonization of protocols as well as methodological or technical prospective are realized under scientifically supervision of our research laboratory. In this context, a study was conducted to evaluate the ability of the NIRS to predict some soil properties: CEC, TOC, TN and clay content. The initial models were elaborated upon local PLS regression on set of 1 300 soil samples. The local PLS calibration used allows an accurate prediction of the soil properties and precision of NIRS technique is comparable to reference analytical metho

Investigating the Effects of Plant Root Exudates on PAHs Bioavailability to Soil Microorganisms in Contaminated Brownfields : Research Methodology.

As a result of heavy industrial past activities, an estimated 6,000 brownfields require remediation in Wallonia. This number rises to over 3.5 million in Europe. Polycyclic Aromatic Hydrocarbons (PAHs) represent 17% of treated pollutants in Wallonia (Aldric et al., 2011). Current remediation techniques are rather expensive and technically demanding (Megharaj et al., 2011). Based on the observation that PAHs soil content decreases in the presence of plants (Cheema et al., 2010), the PhD aims at developing alternative PAHs remediation techniques in brownfields. It is articulated around three research axes. The first axis focusses on plant exudates and how they may improve PAHs bioavailability to soil microorganisms and enhance their degradation. This will be investigated by (i) characterizing several contaminated soils (physico-chemical parameters) and PAH content and factors of bioavailability, (ii) selecting a plant model and collecting root exudates, and (iii) evaluating the effects of exudates on PAHs bioavailability. The objective of the second axis is to evaluate the effects of plant exudates on PAHs degrading microorganisms by (i) comparing PAHs biodegradation in the presence/absence of exudates and (ii) assessing the potential toxic effects of exudate compounds on the microbial communities. The aim of the third axis is to study plant-pollutants interactions by (i) establishing the plant tolerance to several contamination levels and (ii) following PAHs bioavailability when facing real exudation rates, on the field

Combined Application of Organic Amendments and Gypsum to Reclaim Saline-Alkali Soil

peer reviewedSaline–alkali soils have high sodicity, high pH, and high levels of soluble salts, as well as carbonates. This study aimed to evaluate the effect of cattle manure and chicken manure combined with gypsum at three levels on reclaiming a saline–alkali soil, through a soil column experiment. Combined treatments were more effective than those of sole gypsum in reducing the initial exchangeable sodium percentage (ESP) below 5%. Electrical conductivity (ECe) was lowered below 1.6 dS m-1 by all treatments, except the control. The higher effectiveness of manures combined with gypsum can be explained by their synergistic effect on Na+ displacement and subsequent soil structure improvement, leading to an enhancement in the leaching process, and then the salinity/sodicity reduction. Soluble salts and Na+ were considerably reduced in all treatments at the first leaching. Soil ESP and ECe threshold values from the US Salinity Lab classification were reached by any treatment, except the control. The addition of cattle manure or chicken manure might enhance the reclamation effect of gypsum with leaching for some saline–alkali soils

Soil redistribution in rural catchment: how fifty years old soil survey can help model improvement

In a context of high urbanization’s pressure in rural zones, landscape modelling of erosion opens interesting perspectives in land use planning. In most cases, validation data are the weak point. In this study, we present how fifty years old soil observations can help progressing towards a more accurate validation of such modelling in rural areas. As of 1947, a comprehensive systematic survey of the Belgian soil cover was initiated. Field observations were done every 75 meters by soil auger to a standard depth of 125cm (if possible). Map units were delineated on cadastral field survey maps at scale 1:5,000, based on auger observations and landscape context, then generalised on the 1:10,000 topographic base map for a publication at 1:20,000 scale. The legend of the map includes more than 6,000 different soil types and variants. More recently, the Walloon part of this map was digitalised to produce the Digital Soil Map of Wallonia (DSMW). A 10m resolution DEM was build up in 2009. Its RMSE is 0.8m. Soil erodibility and runoff production maps were derived at the same resolution. A land use map exists at 1:10,000 scale since 2005 and is updated yearly. We applied the USPED model (Unit Stream Power - based Erosion Deposition) (Moore and Burch, 1986) in a small watershed where first soil observations took place in 1956. New soil observations were done in 2010. The watershed is completely included in a cultivated area. The model was applied considering a transport capacity limitation proposed by Mitasova and Mitas (1996). Furthermore, we slightly modified it, in order to take into account recent advances in RUSLE factors computations like LS computation proposed by Desmet and Govers (1996) and Nearing (1997). The spatial distribution of erosion and deposition area produced by the model on the basis of the current DEM is consistent with a comparison between old and recent pedological observations. Furthermore, a comparison between horizons’ thickness in 1956 and 2010 gives spatially distributed quantitative information on erosion and deposition. Nevertheless, some uncertainties remain since the pedological descriptions are based on thickness classes, due to the variability of soil cartographical units, and since the current DEM is itself affected by an uncertainty on the elevation value. Future research will then focus on more accurate elevation data as starting point and then it will become conceivable to model the evolution of watershed elevation including land use and other local anthropogenic structures like hedgerows, ditches or grass strips